The increasing prevalence of antibiotic resistance is compromising our ability to treat infection. Therefore, the application's broad, long-term objective is to develop novel therapeutics targeting critical aspects of pathogen host interaction that are predicted to be far less susceptible to anti-microbial resistance. Many bacterial pathogens use specialized secretion systems to inject virulence factors into host cells. These systems are absolutely required for virulence and therefore offer a promising therapeutic target. The aim of this R21 proposal is to establish type IV secretion systems (T4SS) -- one of the major classes of such injection apparatuses - as a target for antimicrobial therapy. Experiments are proposed to: (1) identify T4SS inhibitors using a high throughput screening approach in the model T4SS pathogen, Legionella pneumophila;(2) validate and characterize the specificity of inhibitory activity through several secondary assays of T4SS function;(3) test the prediction that some inhibitors will be broadly active based on homology among T4SS from several agents of public health concern, including CDC priority agents such as Coxiella burnetii;and (4) perform proof of principle in vivo experiments. Taken together these experiments should establish the theoretical basis for T4SS therapy and their potential use as new human therapeutics. PUBLIC HEALTH RELEVANCE: The proposal will identify and characterize new therapeutic agents that target a critical interaction between pathogen and host. We predict these new drugs will be less susceptible to resistance mechanisms that compromise efficacy of traditional antibiotics, and therefore will provide a powerful new therapy for infectious diseases.